The present invention relates to the cryogenic removal of volatile compounds (xe2x80x9cVCxe2x80x9d), especially volatile organic compounds (xe2x80x9cVOCsxe2x80x9d), from a process gas and has particular, but not exclusive, application to the removal of contaminants from a waste process gas stream to meet environmental requirements. It provides processes for removing volatile compounds and apparatus for use in those processes.
Modern industrial processes often produce a gaseous waste stream containing one or more volatile contaminants such as, for example vaporized reactant, product or solvent. Environmental legislation limits the extent to which these contaminants can be released into the atmosphere and several technologies exist to remove them from waste streams. However, with new maximum concentrations being set by environmental bodies as well as more pressure on enforcement, existing technologies often cannot clean waste steams to the desired level in a cost-effective manner.
Process gas streams are cleaned of contaminant(s) chemically by, for example, burning or reaction with another chemical added to, or present in, the process stream, or physically by, for example, condensation. Cryogenic condensation is particularly suitable for removal of a volatile compound, especially a VOC because it permits of the recovery and re-use of the compound. In cryogenic condensation, the process gas stream is cooled to temperatures at which the VC condenses out to form a liquid phase of high VC concentration and a gaseous phase of low VC concentration. The amount of VC left in the gas stream is dependant upon several factors, especially temperature, pressure, process stream composition, and the identity of the VC. In order to meet the relevant legislation, the outlet temperature may have to be well below that required to condense the VC in order to ensure that the remaining VC content of the waste gas is reduced to the required level. With the tightening of legislation, it is probable that, in many cryogenic condensation processes, the outlet temperature will have to be decreased from those currently used. For example; to meet an emission requirement of 20 mg/m3 to recover methyl chloride (CH3CI; freezing point xe2x88x9297.6xc2x0 C.; boiling point xe2x88x9223.7xc2x0 C.) an outlet temperature of xe2x88x92150xc2x0 C. is required and for methylene chloride (CH2CI2; freezing point xe2x88x9297xc2x0 C.; boiling point +40.1xc2x0 C.) an outlet temperature of xe2x88x92120xc2x0 C. is required.
Further, it may be necessary for a cryogenic condensation process to deal with process gas streams having small differences in composition and from which a VC is substantially entirely removed at a common temperature but for which streams there are significant differences in the extent of VC removal at higher temperatures.
The requirement for colder temperatures often causes the VC to freeze forming so-called xe2x80x9cVC icexe2x80x9d. Normally shell-and-tube heat exchangers are used to remove a VC by indirect condensation with liquid nitrogen, or another cryogen, passing on the tube side (inside) of the exchanger, and the VC condensing on the shell side. When freezing takes place, VC ice builds up on the surface of the exchanger tubes, which over time reduces the effectiveness of the exchanger. The normal solution to the problem of VC ice build-up is to have two heat exchangers so that, when one heat exchanger requires regeneration, the process stream can be diverted to the other heat exchanger. Regeneration is by warming to melt the VC ice for removal as a liquid. This xe2x80x98freeze-thawxe2x80x99 type of system is also required when moisture (water vapour) or other compound with a relatively high freeze point is present within the process stream. However, the capital and operating costs are relatively high because of the duplication of equipment and there is a problem of VC ice entrainment as discussed below.
The problem of freezing also can be mitigated by the use of two or more condenser units arranged in series with decreasing operating temperatures. In particular, a first condenser can be provided to pre-cool the waste stream to, for example, +1xc2x0 C., to remove the majority of water before it freezes, and the resultant gaseous stream further cooled in a second condenser with the outlet temperature set to remove the VC. An alternative arrangement is shown in U.S. Pat. No. 5,083,440 where an intermediate heat transfer fluid cooled by the cryogen is used to maintain the process stream temperature above the VC freeze point. U.S. Pat. No. 5,533,338 describes a special cryogenic heat exchanger that utilises a cold, re-circulating, vaporized nitrogen fluid as the refrigeration medium inside the condenser tubes permitting, by careful control of the fluids circulation rate and temperature, the effect of freezing to be minimized.
When cryogenic condensation is used to remove contaminants from a process stream and especially when it is required to cool a solvent to well below its dew-point, fogging can occur within the process stream. When the rate of cooling of a gas exceeds the rate of mass transfer, the bulk of the gas quickly cools to below the dew point of the condensable vapour forming droplets which condense in the process stream without making any contact with a cold surface of the condenser. This fogging can be minimized by using a successive series of cooling stages with increasing temperature control across each step or by using a de-mister to capture the droplets within the condenser, thereby preventing droplets being than entrained in the process gas stream.
When operating at very low temperatures, small particles of VC ice often are entrained in the cleaned process gas exiting a cryogenic condenser, thereby increasing the retained VC content above the design level of the condenser. As VC ice builds up on exchanger tubes, the outer layer of the ice is like a fine powder and is easily picked up and entrained by the process gas stream passing over it. If the process gas velocity is high, some VC ice can become entrained in the gas stream, especially if the residence time in the exchanger is too short. Further, the strength of adhesion of VC ice to the exchanger tube can be low if, for example, the rate of ice formation is so rapid that, as the ice forms on a cold surface, contraction takes place and the bond that causes the ice to stick to the surface is broken.
Systems installed to prevent the entrainment of VC ice in the cleaned process stream typically have been liquid separation devices such as wire mesh de-misters and/or liquid separators. Although these systems work well for removal of liquid droplets, they are ineffective with fine (less than 100 xcexcm) particles and cause pressure drop problems in the system due to VC ice blockage. Further, de-misters are installed in the end of the condensers, after the tube bundle, and cannot be easily kept cold or defrosted.
It has long been well known to produce coffee aroma frost by use of a cryogenic condenser to condense volatiles from an aroma-bearing gas produced during coffee processing (see, e. g. U.S. Pat. No. 2,680,687 published Jun. 8, 1954). The gas typically is mainly carbon dioxide (up to 90 wt % or more) together with water vapour and the aromatic constituents responsible for the aroma.
GB-A-1,339,700 (published Dec. 5, 1973) disclosed the use of a scraped-wall condenser in which aroma frost is continuously or periodically scrapped from a cryogenically cooled condenser wall on which it builds-up.
GB-A-1,480,997 (published Jul. 27, 1977) disclosed using a filter element to accumulate sublimated aroma frost particles entrained in the gaseous phase remaining after condensation. The resultant layer of aroma frost particles built-up on the filter element is stated to minimize passage of uncondensed aromatics directly to the atmosphere as well as minimising the loss of frost particles. The element can be located internally or externally of the condenser and can be freed of frost by shaking or vibrating the filter.
U.S. Pat. No. 5,182,926 (published Sep. 16, 1991) discloses directly contacting coffee or other sublimated aroma-bearing gas with liquid nitrogen to form a suspension of aroma frost particles in gaseous nitrogen. The suspension is passed through a tubular porous filter to collect the sublimated aroma frost particles, which are dislodged from the filter by periodically back pulsing the filter.
It also is known from, for example, U.S. Pat. No. 4,317,665 (published Mar. 2, 1982) to separate water ice from recycle air leaving a cryogenic food freezer by passage of the air through a bag filter which is periodically pulsed to dislodge ice collected on the bag.
The object of the present invention is to improve the efficacy with which cryogenic condensation can be used to remove VC contaminants from process gas streams. In particular, in a first embodiment of the present invention, the object is to reduce the VC content of the treated stream remaining after liquid condensation of the VC content thereof with accompanying VC ice formation. It is an object of a second embodiment of the invention to reduce the VC content of the treated stream remaining after condensation of the VC content in a vortex of liquid cryogen, e.g. liquid nitrogen.
It has been found that the primary object of the invention can be achieved in a relatively simple and cost effective manner by provision downstream of the condenser of a filter to remove at least particles of a size of greater than about 50 xcexcm entrained in the treated waste stream.
According to an embodiment of one aspect, the present invention provides a cryogenic process for the removal of a volatile compound (xe2x80x9cVCxe2x80x9d) from a process gas stream comprising cooling the gas stream in a condenser to condense the VC to form both liquid VC and VC ice and provide a treated process gas which is essentially freed of the VC but contains entrained VC ice particles. The improvement consists of passing said treated process gas through a filter downstream of the condenser to remove at least said entrained VC ice particles which are of a size greater than about 50 xcexcm.
According to another embodiment of this aspect, the present invention provides a cryogenic process for the removal of VC from a process gas stream comprising cooling the gas stream within a condenser to condense the VC to form VC ice and provide a treated process gas which is essentially freed of the VC but contains entrained VC ice particles. The improvement consists of cooling the gas stream in a vortex of liquid cryogen and passing the treated process gas through a filter downstream of the condenser to remove at least said entrained VC ice particles which are of a size greater than 50 xcexcm.
The present invention also provides an apparatus for the cryogenic removal of a volatile compound (xe2x80x9cVCxe2x80x9d) from a process gas stream by a process of the first aspect, said apparatus comprising:
a condenser for cooling the gas stream to form both liquid VC and VC ice and providing a treated process gas which is essentially freed of the VC but contains entrained VC ice particles;
means for removing said liquid VC from the condenser;
means for removing sa id VC ice from the condenser;
means for removing said treated process gas from the condenser; and
filter means downstream of the condenser to remove at least said entrained VC ice particles which are of a size greater than about 50 xcexcm from the treated process gas stream.
According to another embodiment of this aspect, the present invention provides an apparatus for the cryogenic removal of a volatile compound (xe2x80x9cVCxe2x80x9d) from a process gas stream by a process of the first aspect, said apparatus comprising:
a condenser for cooling the gas stream in a vortex of liquid cryogen to form VC ice and providing a treated process gas which is essentially freed of the VC but contains entrained VC ice particles;
means for providing a vortex of liquid cryogen within the condenser; and
filter means downstream of the condenser to remove at least said entrained VC ice particles which are of a size greater than 50 xcexcm from the treated process gas stream.